1. Field of the Invention
This invention relates to a surface light source device of side light type and a light control element for use therein. This invention is applied to for example, back lighting of a liquid crystal display apparatus.
2. Description of the Related Art
A surface light source device of side light type has been employed in, for example, a liquid crystal display apparatus. This device irradiates the liquid crystal display panel from its back. This disposition is suitable for thinning the entire shape of the apparatus.
In the surface light source device of side light type, ordinarily, a rod-like light source such as cold cathode tube is employed as a primary light source and disposed on a side of the light guide plate. Irradiation light emitted from the primary light source passes a side end face of the light guide plate so that it is introduced into the inside of the light guide plate. The introduced irradiation light is transmitted inside of the light guide plate. In that process, light emission occurs from a major surface of the light guide plate toward a liquid crystal display panel.
As the light guide plate employed in such a surface light source device of side light type, a type having approximately equal plate thickness and another type in which the plate thickness decreases as it goes far from the primary light source have been well known. Generally, the latter emits irradiation light more effectively than the former.
FIG. 21 is a disassembly perspective view showing the surface light source device of side light type using the light guide plate of the latter. FIG. 22 shows a section along the line A--A of FIG. 21. Referring to FIGS. 21, 22, the surface light source device of side light type 1 comprise a light guide plate 2, a primary light source 3 disposed on a side thereof, a reflection sheet 4, a light diffusing sheet H, and prism sheets 5, 6 acting as a light control member. The reflection sheet 4, light guide plate 2, light diffusing sheet H and prism sheets 5, 6 are laminatedly arranged.
The light guide plate 2 is a transparent light guide plate or light scattering guide plate having a wedge-shaped section. The former is a light guide plate made of, for example, transparent acrylic resin and usually a light scattering surface is formed on the back surface 2B. The latter is a light guide plate made of light scattering substance. The light scattering substance is composed of, for example, matrix made of poly methyl methacrylate (PMMA) and a great number of light transmissive fine particles uniformly diffused therein. Refractive index of these particles is different from that of the matrix.
The primary light source 3 has a cold cathode tube (fluorescent lamp) 7 and a reflector 8 which is disposed on the back having an approximately semi-circular section. The irradiation light is supplied through an opening of the reflector 8 toward a side end face of the light guide plate 2. The reflection sheet 4 is made of sheet-like regular reflection member made of metallic foil or the like or sheet-like irregular reflection member made of white PET film.
The irradiation light L from the primary light source 3 is introduced into the light guide plate 2 through the incidence surface 2A which is a side end face of the light guide plate 2. The reflection light L is repeatedly reflected between the back surface 2B on which the reflection sheet 4 is laid and the emission surface 2C and then transmitted toward a distal end thereof. In this while, the irradiation light L is subjected to scattering action by the back surface having light scattering property and fine particles inside of the light guide plate 2. If the reflection sheet 4 made of irregular reflection member is employed, the irradiation light is also subjected to irregular reflection action.
Each time when the irradiation light L is reflected by the slope 2B, the incidence angle relative to the emission surface 2C gradually decreases. The decrease of the incidence angle increases component having a smaller angle than the critical angle relative to the emission surface thereby inducing emission from the emission surface. As a result, shortage of the emission light in a region far from the primary light source 3 is prevented.
Because the irradiation light emitted from the emission surface 2C has experienced scattering of light by the back surface 2B having the light scattering property and light guiding fine particles, and irregular reflection by the reflection sheet 4, it has a characteristic of the scattered light. However, main propagation direction of the irradiation light emitted from the light guide plate 2 is inclined toward the distal end direction (opposite direction to the primary light source 3) relative to the frontal direction. That is, the emission light of the light guide plate 2 has directivity. Such a characteristic of the light guide plate is called emission directivity.
The prism sheets 5, 6 are disposed to correct the emission directivity of the light guide plate 2. The light diffusing sheet H is disposed as required, to prevent the light scattering surface of the back surface 2B from being noticed from above the light emission surface 2C by scattering the irradiation light emitted from the light guide plate 2 and further to suppress local highlighting, shading and the like of the light guide plate 2. If the light guide plate 2 is a light scattering guide plate, often the light diffusing sheet H is omitted.
The prism sheets 5, 6 are formed of transmissive sheet-like members of polycarbonate or the like. The prism surface is formed on an opposite side to a surface (outside surface) opposing the light guide plate 2. The prism surface has a great number of projections which extend approximately parallel in a single direction and have a triangle section. In the inside prism sheet 5, its projections are oriented so as to run parallel to the incidence surface 2A. The outside prism sheet 6 is oriented so that the projections extend approximately perpendicular to the incidence surface 2A.
The slopes of these projections correct main emission direction of the emission light to the frontal direction of the emission surface 2C. A double-sided prism sheet having each prism surface on both sides may be used.
Generally, as compared with the surface light source device of side light type in which an approximately equal thickness light guide plate is employed, the surface light source device of side light type employing such a wedge-shaped light guide plate and a prism sheet is capable of emitting the emission light more effectively to the frontal direction.